<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }
    body {
      height: 100vh;
      display: flex;
      justify-content: center;
      align-items: center;
      background-color: #000;
    }
    a {
      position: relative;
      display: inline-block;
      width: 200px;
      height: 60px;
      line-height: 60px;
      text-align: center;
      color: #fff;
      font-size: 24px;
      text-decoration: none;
      background: linear-gradient(90deg, #03a9f4, #f441a5, #ffeb3b, #03a9f4);
      background-size: 400%;
      border-radius: 10px;
    }
    a:hover {
      animation: animate 8s linear infinite;
    }
    @keyframes animate {
      0% {
        background-position: 0 0;
      }
      100% {
        background-position: 400% 0;
      }
    }


    a:before {
      content: '';
      position: absolute;
      left: -5px;
      top: -5px;
      right: -5px;
      bottom: -5px;
      z-index: -1;
      background: linear-gradient(90deg, #03a9f4, #f441a5, #ffeb3b, #03a9f4);
      background-size: 400%;
      border-radius: 10px;
      opacity: 0;
      transition: all 1s;
    }
    a:hover:before {
      filter: blur(20px);
      opacity: 1;
    }
  </style>
</head>
<body>
  <a href="#">Button</a>
</body>
</html>